In a cellular communication system, the coverage area is divided into a plurality of cells. Each cell is served by a base station that communicates with mobile stations within that cell. The base stations are connected to a Mobile Services Switching Center (MSC) which, in turn, is connected to the Public Switched Telephone Network (PSTN). The MSC switches calls to allow mobile users to communicate with other mobile users or to telephones connected to the PSTN.
Cellular communication systems are designed to handle a very large number of mobile users with a limited number of available frequencies. To accommodate the large number of users, the available frequencies are subdivided into frequency groups. Typically, one frequency group is assigned to each cell in the network. The frequency groups are assigned such that adjacent cells do not use the same frequency group. Cells to which the same frequency group has been assigned are called “co-channel cells.”
It is common practice to utilize sectored cells to reduce interference between co-channel cells. For example, a cell may be subdivided into three 120° sectors. The frequency group assigned to a particular cell is then further subdivided into subgroups which are assigned to respective sectors in each cell. In the past, the channel allocations to sectors in a cell are fixed. That is, each sector within the cell is assigned a fixed number of channels. One consequence of the fixed allocation scheme is that it reduces the spectral efficiency of the cellular network when one sector is fully loaded and another sector within the same cell has free channels. In a cellular system without sectorized cells, the base station would simply assign the next available channel. However, in the case of the sectorized cell, there would be no available channels to assign in the fully-loaded sector, even though there are unused channels in the same cell.